Ayako Haibara is a hostile female spirit in Fatal Frame IV: Mask of the Lunar Eclipse. She was one of the many patients that suffered from Luna Sedata Syndrome. She was being treated inside of Rougetsu Hall, a sanatorium built on Rougetsu Island specifically to attend to patients of Luna Sedata Syndrome. She is also one of the patients that knew the heroines of Fatal Frame IV during their stay in Rougetsu Hall. Ayako stood out from the other patients due to her homicidal tendencies. Eventually she met her end when the disease could not be controlled and she finally bloomed.

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Ayako was hospitalized at Rougetsu Hall when she was 12 years old, after displaying sudden symptoms of memory impairment. She was referred by the hospital Director, Shigeto Haibara, and therefore was treated with respect by the hospital staff, although her personality was destructive and sadistic. Her last name was purposefully not mentioned to other patients, so as not to reveal who her blood relations were. Ayako's family name may also have been the reason why her antics were tolerated at the hospital. She had a fascination with killing and causing harm to others, and would often purposely injure the nurses and other children. She bullied her fellow patient Madoka Tsukimori viciously, beating her up and pushing her down the stairs, and even cut off the head of Madoka's pet canary with a pair of scissors.

On the Day Without Suffering, Ayako saw a man Blooming outside. She retreated to the room with the canopied bed that was connected to her normal room, and waited there for somebody to help her. But on that exact same day she finally bloomed.

She's a young girl in a red/dirty golden Victorian-like dress with lace on the collar and the bottom of her skirt. She has long black hair with bangs and gold colored eyes. It's unknown what her eye color was before she bloomed.

she sees Ayako dragging her nurse Fuyuko Shiratsuki towards her room. Misaki takes a picture and follows her inside. Misaki looks around for Ayako, who is sitting behind the lamp near the bed. Misaki turns around to look to the lamp but Ayako quickly jumps at her. Misaki uses the camera to fight her.
Later in Chapter 4 Ruka goes into the Second Floor Management Office to

Ayako behind Misaki in her second room

see the list of names. It beeps and Ayako's voice is heard; Ruka then goes straight to room 207 and fights Ayako.

She reappears in Chapter 5 in the hidden room that is connected to hers. This time she appears behind Misaki and another fight occurs.

Ayako was kept in Room 207 - Rangetsu (蘭月). This can be read as "orchid moon" or "month of orchids."

It's been theorized, but never proven, that Ayako is the product of an incestuous union between Sakuya and You. [citation needed]

In a notebook entry written by You, it's mentioned that Shigeto compared Ayako's tendency towards "excessive mischief" to You's own, which You apparently approved of.

You wrote of Ayako often in his notebook. His and Ayako's rooms are also connected.

In Sakuya's diary, she oddly addresses Ayako by name along with her father and brother, possibly implying that they are at least related.

Ayako being the daughter of at least one of his children or even being some other blood relative may be a reason why Shigeto didn't want her family name disclosed, especially if she was the daughter of both You and Sakuya.

Furthermore, Ayako being a relative (either granddaughter or some other blood relation) could be a reason why she was given preferential treatment, such as with her room and the tolerance of her sadistic activities. Sakuya received similar treatment, notably.

In another diary entry, Sakuya expresses a desire to guide Ayako and implies that the two share a bond much like mother and daughter, as evidenced by her parallel to herself and her own mother.

You implies in a notebook entry that he wants to live together with Sakuya and an unnamed person. This third person could possibly be Ayako.

It should be noted that incestuous implications have appeared in other Fatal Frame titles, which could lend credence to this claim.

BGM for Room 207

Ayako is the only patient in Rougetsu Hall that had two different rooms. These rooms also had two distinct background themes:

In Ayako's main room, Room 207, her laughter can be heard in the background as well as some ghoulish moaning, alluding to her antisocial behavior and enjoyment of others' pain.

BGM for Room with Canopied Bed

In her second room, the Room with Canopied Bed, no sound can be heard aside from her strained breathing. These whimpers are meant to depict how she was tied down on the bed and how she struggled to fight the disease before blooming.